The present application relates to a method for determining characteristics of the flow of a medium in a channel by transmitting and receiving sound waves along one or more acoustic paths using one or more acoustic transducers, each of which can act individually as a transmitter and receiver, and by measuring the transit times of the transmitted sound waves, and determining flow characteristics from the measured transit times.
Such a method is generally known. It involves determining the average flow velocity and/or the throughput of the medium from the difference in transit time of sound waves which are transmitted in the downstream direction and upstream direction respectively between acoustic transducers set up at a distance from each other. The medium can be a gas or liquid.
The transit times of the sound waves not only depend on the flow velocity of the medium, but are also influenced by the flow profile of the medium. In a flowing medium, apart from the longitudinal movement, a swirl can occur. Other possible disturbances of the ideal flow profile are a flow velocity fluctuating in time and a flow profile which is asymmetrical relative to the longitudinal axis. Such disturbances occur in particular in pipe systems which have a complex structure.
The ultimate reliability of the calculated flow velocity of the medium depends on the distance covered, the acoustic path, on the transmitted sound wave, and on the calculation method used. Many configurations are known for the acoustic path.
In the case of the conventional methods which are used in commercially available measuring instruments, several acoustic paths are used, running parallel to each other. The known numerical Gaussian square method is used for positioning of the paths and the various weighting factors which are allocated to the measured velocities.
The advantages of this method are clear. No additional information on the flow profile is required for calculating the velocity. The weighting factors are fixed in advance, so that the microprocessor which is used for calculation of the characteristics need only carry out a limited number of calculations.
Although this means that measurement of the flow velocity is simple to carry out and excellent results can be obtained in ideal flow conditions, the method has a number of clear disadvantages, due to the underlying assumptions and limitations of the Gaussian integration method. These underlying assumptions and limitations are as follows:
It is assumed that the flow profile is fully axially symmetrical. PA1 No additional information from the Reynolds number is used. PA1 As a result of the fixed weighting factors for the various acoustic paths, which are optimized for the undisturbed flow profile, errors will occur when the actual flow profile deviates from the ideal profile.